The invention relates to hydraulic control apparatus for an automatic dual clutch transmission and to a method for controlling said the transmission with the hydraulic control apparatus.
Dual clutch transmissions have been known for some time and are preferably applied in motor vehicles. Generally, a dual clutch transmission comprises two partial transmissions, each one of which having a separate clutch and a group of gears. For the reason of a significant mechanical complexity required for actuating the clutches as well as shifting gears in the two partial transmissions, dual clutch transmissions are usually designed as automatic transmissions, i.e. both actuating the clutches as well as shifting the gears is a done by means of servo drives that can for instance be designed as hydraulic shifting means that are controlled by a hydraulic control apparatus.
From EP 1 635 091 A1 the hydraulic control apparatus for an automatic dual clutch transmission is known comprising a first clutch with a first partial transmission and a second clutch with a second partial transmission as well as a shifting system for shifting into and out of gears in the two partial transmissions. The hydraulic control apparatus comprises a control unit predetermining at least one of the parameter's control pressure and controllable volume flow for activating the first and second clutches and for actuating the shifting system. Further, the control apparatus comprises a changeover means provided between the control unit and two clutches and/or the shifting system by means of which the control pressure and/or the controllable volume flow are directed to the clutches or the shifting system. The control unit comprises a first control valve and a second control valve, while the changeover means comprises a first changeover valve and a second changeover valve. In a first position AI, the first changeover valve connects the first control valve with the first clutch and separates the first control valve from the shifting system. In position BI, the first changeover valve connects the first control valve with the shifting system and separates the first control valve from the first clutch. Also the second changeover valve is capable of assuming two positions, wherein in a first position AII the second control valve is connected with the second clutch and the second control valve is separated from the shifting system. In a second position BII, the second changeover valve provides a connection between the second control valve and the shifting system while it separates the second control valve from the second clutch. The control apparatus according to EP 1 635 091 A1 enables shifting of gears by means of only one of the two control valves, respectively.
For being able to actuate both clutches under emergency running conditions, EP 1 635 091 A1 suggests that in the position BI of the first changeover valve and in the position BII of the second changeover valve the first control valve is connected with the second clutch and the second control valve is connected with the first clutch. If for instance the first control valve fails, the first clutch can be controlled by the second control valve if the first and the second changeover valves are in positions BI and BII, respectively. The second clutch can also be continuously operated by the second control valve, if said second control valve is shifted into the position AII.
Under emergency running conditions, severe problems might occur if one of the control valves fails while in an open position. In that case, the failing control valve provides an undesired pressure rendering it difficult or impossible to open one of the clutches for disengaging one of the gears that is assigned to the respective partial transmission. Furthermore, the changeover valves for this solution require numerous input and output ports, making these changeover valves of complicated design and accordingly, requiring a lot of space.